The enrichment of industrial composts with useful microbial flora is of interest either for decontaminating plant growing substrates, or for improving the fertility thereof.
Various microorganisms and various bacteria are used as plant biocontrol agents.
Plant growth promoting microorganisms are called PGPMs. Symbiotic mycorrhizal fungi and certain species of Trichoderma that are mutually beneficial with plants are part of this category. Plant growth promoting rhizobacteria are called PGPRs. This category comprises, for example, useful Pseudomonas and Bacillus. 
The Trichoderma genus is an agronomically important group since it comprises biocontrol-agent fungi, the mode of action of which has been the subject of a great deal of work. Recent studies in fact mention that this filamentous fungus has a capacity to intervene according to various mechanisms: mycoparasitism, antagonism (competition), antibiosis (production of antibiotics), root stimulation, growth stimulation by solubilization of fertilizing minerals, stimulation of plant natural defenses.
Fungi of the Trichoderma genus belong to the phylum Ascomycota, class Ascomycetes, family Hypocreales. They are microscopic fungi of which there are terrestrial species and marine species. They are found in decomposing wood, in plant residues and in all soils (forest humus, agricultural earths) (from 10 to 10 000 propagules/g in temperate or tropical soils). They colonize the roots of herbaceous and ligneous plants without any damage. In addition, this fungus can penetrate roots and promote development, nutrition and resistance to diseases. The species atroviride is one of the common species of Trichoderma. 
Root colonization by Trichoderma sp. can increase root growth and development, crop yield, resistance to abiotic stresses and nutrient absorption and use (Harman G. E. et al., Nature Reviews/Microbiology, 2, 43-56, 2004).
When they exist, the effects of growth stimulation originate from the direct action of Trichoderma on plants and are not directly linked to antagonisms with pathogens. These effects are visible both on nondisinfected growing substrates and on sterile substrates. The growth stimulation mechanisms are poorly elucidated and could be due to the suppression of oxidative damage on the roots, to the secretion of growth factors by the fungus, to the inhibition of bothersome microflora and to the improvement of micronutrient transport.
The effects are unequal from one strain to another. Some strains have growth-stimulating effects, but others have inhibitory effects (for example, Trichoderma viride RF1, J. G. Menzies, Plant Pathology, 42, 784-791, 1993).
The antagonistic properties of Trichoderma are better documented than the stimulatory properties. The antagonistic potential of Trichoderma with respect to numerous pathogenic soil fungi of plants was discovered in the 1930s (Weindling R., Trichoderma lignorum as a parasite of other soil fungi, Phytopathology, 22, 837-845, 1932). The most obvious application is biological control in agriculture (biocontrol), including in biological agriculture where (EC) regulation No. 2092/91 provides for this use.
Generally, there are two ways to enrich a medium with microorganisms:                by dilution in the medium of a sufficient amount of microorganisms from a concentrated preparation; or        by inoculation of the medium and in situ multiplication of the microorganisms.        
The first technique calls for production of microorganisms in a separate system. It applies well to microorganisms which have stable resistance forms (for example, spores, conidia or chlamydospores). The technique consists in diluting the microorganism obtained separately in a medium which must be compatible, and which may be capable of aiding its regeneration when a favorable condition occurs (for example, hydration, heating of the medium). Such a technique is described, for example, in patent application US 2004/0136964A1. Said document relates to a substrate containing a strain of Trichoderma asperellum for the biological control of Fusarium and of Rhizoctonia, said substrate being obtained from a mixture consisting of waste, purification sludge, peat, bark or compost.
The second technique applies to microorganisms capable of multiplying in the medium until exhaustion of the nutritive resources or degradation of the vital conditions, so as to subsequently give dormant forms termed resistance forms. The resistance forms which can regenerate the microorganisms fall into the category of propagules. This second technique makes it possible to obtain a medium rich in stable propagules. The exhaustion of the readily fermentable matter makes the medium not very suitable for the development of other microorganisms. The enrichment by in situ multiplication can apply to bacteria, yeasts and fungi, capable of living in wood fiber-based media. The multiplication also relates to the category of mycorrhizal fungi which are also saprophytes. The multiplication may also relate to symbiotic mycorrhizal fungi of plants, on condition that they are cultivated on host roots.
Contrary to the dilution technique, the in situ multiplication does not require large amounts of microorganisms produced under sterile conditions. The multiplication is obtained by virtue of the consumption of nutritive substances contained in or added to the substrate. The multiplication is, in principle, aerobic. The medium is considered to be “activated” when specific nutritive substances are added in order to promote multiplication. The specific nutritive substances added to the “activated” medium are partially or totally consumed by the end of the multiplication phase. This is reflected by a loss of mass and the production of volatile metabolites, in particular of carbon dioxide (CO2) and of water (H2O).
The in situ multiplication technique comprises a phase of preparing the medium, with if necessary a disinfection, the addition of a sufficient amount of primary inoculum of microorganisms, the microorganism multiplication period, the appearance of the resistance forms and the maturation. Each step has a variable duration depending on the conditions and the microorganisms used. The multiplication can be carried out via successive enrichment cycles, the product of a multiplication cycle serving to inoculate a larger amount of medium. Cycle after cycle, the successive multiplication media can become increasingly simple and the multiplication conditions less and less severe. This makes it possible to obtain a good yield without using laborious production techniques. The stepwise technique improves the biomass yield and reduces the loss of substrate consumed for the growth of the microorganisms.
The in situ multiplication also makes it possible to reduce the bothersome microorganisms during the production of the compost by virtue of the introduction of competition. The propagules obtained by the multiplication technique are more stable than microorganisms added after the production of the compost. The compost obtained by means of the multiplication process may contain secondary metabolites released during the fermentation. The secondary metabolites of certain species of fungi (Trichoderma) and bacteria (Pseudomonas) are useful to plants, either because they decontaminate and detoxify the substrates, or because they stimulate growth.
In practice, the microorganisms that are useful to plants live in the rhizosphere or in the immediate proximity of the rhizosphere and benefit, for their nutrition, from the root exudates and the plant residues. Certain microorganisms, such as Trichoderma, live saprophytically in the soil or at the surface, on dead plant remains.
Various sorts of fibers are favorable to the growth of microorganisms; mention may in particular be made of wood fiber, palm fiber and the various plant fibers used in agricultural, textile or insulation applications. Aerated and fibrous materials, of plant origin, made of shavings, leaves or straw, are also suitable.
Patent applications EP-A-0 147 349 and FR-A-2 776 470 describe a culture medium based on wood fibers obtained by steam cooking and defiberizing, according to processes encountered both in the paper-making industry and in the chipboard industry. As indicated in patent application FR-A-2 776 470, the defiberizing is conventionally carried out by means of a disk grinder, using wood shavings from which the fibers are “extracted” by shearing at high temperature.
However, it has been noted by the applicant, following numerous tests, that the methods described in the abovementioned documents have a tendency to cause cooking of the wood similar to the beginnings of roasting or to thermostabilization, the consequence of which is a reduction in the hydrophilicity and in the water retention capacity of the wood fibers produced. These drawbacks make the wood fibers not very conducive to an appropriate multiplication of microorganisms.
It is therefore desirable to be able to have wood fibers capable of promoting the multiplication of microorganisms, in particular of the Trichoderma genus, which are free of the drawbacks mentioned above.
It is also desirable to be able to have a culture medium which is suitable for multiplying such microorganisms, and which makes it possible to obtain a growing support conducive to the growing of plants.